Source:

Response:

The Castendolo bones belong to skeletons of several men, women, and
children. They are a recent burial in Pliocene sediments, evidenced by
the fact that other fossils but not the human bones were impregnated
with salt.

The Olmo skull is from upper Pleistocene gravel, placing it in the
Upper Paleolithic (Stone Age) period. It is not out of place.

The Calaveras skull was a deliberate hoax and was immediately
recognized as such by scientists (Conrad 1982).